1. Field of the Invention
This invention is directed to seismic data acquisition and recording systems and methods; in one aspect, to such systems and methods for recording in a useful and meaningful way all the seismic data related to one or more seismic events, in one aspect without regard to, or synchronization with a navigation system and/or energy source control system; and, in one aspect, to such systems and methods in which acquired data is associated with seismic events at some point in time after the data is acquired.
2. Description of Related Art
In a variety of prior art seismic data systems, only a selected portion of generated seismic signals are recorded. As shown in FIG. 1 a prior art seismic system S is tightly coupled to the actions of a navigation system N and an energy source controller E. The seismic system S includes a data acquisition subsystem A which interfaces with apparatus on seismic cables C and which is slaved to the action of systems N and E. A recording subsystem R is in turn slaved to the data acquisition subsystem A.
The navigation system N receives time and positioning information from a Global Positioning System ("GPS") G. This information indicates the location of the navigation system N. In existing systems the seismic system S and the energy source controller E may be located on separate vessels. Components of the system N and/or of the System G may be distributed over several vessels. These systems provide for the exact positioning of all vessels involved in a seismic survey and, therefore, the physical location of interrelated systems S and E.
In these prior art systems, the navigation system N acts as a master controller of subsystem E and subsystem A, i.e., the system E and subsystem A are, in this sense, dependent on and not independent of the master controller navigation system N. Also, the subsystem E and subsystem A are co-dependent, i.e., they send signals, information, and commands to each other and, in certain instances, do not act without them.
In a typical operation of the system of FIG. 1, the master controller navigation system N, constantly receiving time and position signals from the GPS system G, sends a "start" command to the acquisition subsystem A and, substantially simultaneously, a "start" command to the energy source controller subsystem E. Based on known location, these "start" commands are sent prior to the arrival at a known desired firing point of the system and of the energy source. This pre-firing location (at which initial "start" commands are issued) is chosen so that the E and A subsystems have time to accomplish certain tasks and complete firing of the energy sources at the desired known firing point.
The acquisition subsystem A upon ascertaining that it is ready to acquire and record data, indicates its ability to start an acquisition cycle by issuing a "Fire" command to the energy source controller system E after a fixed delay from the receipt of the "start" command from the navigation system N. The fixed delay is referred to as an Acquisition Synchronization Time and is typically 200 milliseconds. After the system E receives the "Fire" command from subsystem A, the source controller system E sequences its energy sources (typically air guns) in such a manner that the peak power of the array of energy sources is at a maximum at a fixed point in time following the receipt of the Fire command. At this point in time (referred to as TO or Time Break) the energy source controller E issued a Time Break command to the acquisition subsystem A. The time interval from the receipt of the Fire command and issuing of the Time Break command is referred to as Gun Synchronization Time and is typically 128 milliseconds. The subsystem A then begins to acquire data generated by the firing of the seismic energy sources for a fixed period of time (typically 6-8 seconds) referred to as the Acquisition Time. At the completion of the Acquisition Time the acquisition subsystem A will typically inhibit the acceptance of any other start commands from the navigation system N while it logs status and initializes acquisition system components for the next acquisition cycle. This time, which is referred to as the Acquisition System Overhead, typically lasts for approximately 500 milliseconds. During this time the acquisition subsystem A is acquiring and logging status concerning the performance of the acquisition subsystem, synchronizing configuration changes with the multiple CPU's that are contained within the acquisition subsystem and distributing parameters to these various CPU's that will control the actions of the acquisition subsystem A during the next acquisition cycle. Optionally the acquisition subsystem A will start the recording subsystem R process that transfers the acquired seismic data to long term storage on magnetic tape. The acquisition subsystem A does not typically wait for the completion of the recording phase before it is once again able to accept system starts from the navigation system.
If a new start command is issued to the acquisition subsystem A at any point in time during the Acquisition Time or during the Acquisition System Overhead Time the acquisition subsystem will ignore the start command and the next acquisition cycle will not be started. Since this can be a very serious problem, typical operating procedures dictate that the speed of the shooting vessel be adjusted to guarantee that the time interval between consecutive start commands from navigation system N be greater than the sum of the Acquisition Synchronization Time, Gun Synchronization Time, Acquisition Time, and the Acquisition System Overhead has elapsed.
The time interval between consecutive start commands from navigation system N is a function of the speed of the vessel between the fixed locations at which the acquisition subsystem A must be cycled. These fixed locations are known as Shotpoints. The speed of the vessel through the water is, in turn, a function of the vessel propulsion as well as numerous environmental elements that cannot be controlled. These include wind speed and direction, ocean current, and wave action. In order to compensate for these variables a Cycle Delay Time is introduced after the end of the Acquisition Time to insure that the next Start command from the navigation system N is not issued prior to the completion of both the Acquisition Time and the Acquisition System Overhead Time. This Cycle Delay must be greater than the Acquisition System Overhead and allow for the effect on vessel speed caused by environmental elements. Typical values for the Cycle delay are 1.25 seconds.
As shown in FIG. 2 the System Dead Time is the sum of the Cycle Delay, Acquisition Synchronization Time, and Gun Synchronization Time and is typically greater than 1.5 seconds. The productivity of prior art seismic systems is severely reduced by the need to support the System Dead Time interval. For an Acquisition Cycle Time of 6 seconds the effect of the additional 1.5 seconds of dead time reduces production by approximately 25%. Assuming 12.5 Meter Shotpoint intervals the boat speed in this case must be reduced from 4 knots to 3.24 knots to compensate for the System Dead Time. At this speed the cable may become uncontrollable forcing the survey to be done in two passes over the same area with the first pass collecting every other Shotpoint and the second pass collecting the Shotpoints skipped in the first pass. In this case the effect of the System Dead Time reduces production by 50%.
For the interdependent system N, E and subsystems A to work correctly their operations must be temporally synchronized since the systems are interrelated and co-dependent; i.e. they operate and function together in real-time and must do so to be effective in order to implement the temporal synchronization, delays are introduced in the system operation that adversely affect production. During these delay/synchronization periods seismic data is being generated (e.g. by reflections and refractions of a generated acoustic wave field from an earth layer) but must be discarded due to the synchronization delays inherent in the closely coupled system formed by systems N, E, and subsystem A.
There has long been a need for seismic data methods and systems that have reduced or no deadtime, i.e., a system in which most or all generated data is recorded and is, therefore, potentially useful. There has long been a need for seismic data methods and systems in which real-time system/subsystem synchronization are not required.